1. Field of the Invention
This invention relates to a control device for an internal combustion engine of the fuel injection type, and in particular to a control device for a two-cycle fuel-injection internal combustion engine that is relatively small in size and suited for use in a portable working machine such as a chain saw or a bush cutter.
2. The Prior Art
There is known a two-cycle internal combustion engine of the direct fuel injection type, as disclosed, for instance, in U.S. Pat. No. 4,813,391. According to this technique, a fuel injection nozzle is disposed at a cylinder head portion of a combustion chamber so as to permit the injection of fuel to be effected directly into the combustion chamber. In this case, a fuel pump is actuated taking advantage of the fluctuation in pressure within a crank case. Through such actuation of the fuel pump, a fuel is fed to the fuel injection nozzle and then directly injected into the combustion chamber. The stroke of fuel injection, i.e. the injection of fuel from the injection nozzle, is performed as follows. As the piston is moved downward, the pressure inside a crank-case is proportionally increased, and when the piston is brought down to approximately the bottom dead center, the fuel pump is actuated by the pressure inside the crank-case, thus feeding fuel to the fuel injection nozzle, from which the fuel is injected by the pressing force of the fuel pump into the combustion chamber.
There is also known another example of a two-cycle internal combustion engine of the direct fuel injection type, wherein a fuel injection nozzle is disposed midway of an air passage (scavenging passage) communicating with a combustion chamber. Fuel from the injection nozzle is injected toward a heat conductive wall constituting a cylinder wall and impinged thereon so as to be gasified and mixed with air passing through the air passage, the resultant air-fuel mixture being fed to a combustion chamber (U.S. Pat. No. 4,876,999).
With increasing concern in recent years of environmental problems, a reduction not only of the toxic substances in the exhaust gas but also engine noise is strongly demanded, even in a small two-cycle internal combustion engine. In particular, there are problems inherent to a two-cycle internal combustion engine, i.e. a problem of how to reduce the quantity of HC in the exhaust gas which is brought about due to a phenomenon of blow-by of unburnt air-fuel mixture from the combustion chamber, and a problem of how to prevent the discharge of unburnt air-fuel mixture which is brought about due to a phenomenon of spitting of fuel toward the air-cleaner. These problems are also desired to be solved.
On the other hand, a lean burn engine or a direct injection engine wherein fuel is directly injected into a combustion chamber has been recently proposed as a four-cycle internal combustion engine. According to these types of engine, the reduction of toxic substances in the exhaust gas by means of a lean-burning technique is taken into consideration. Therefore, it is now studied how to apply this technique to a two-cycle internal combustion engine.
In the case of the aforementioned engine of the direct fuel injection type (U.S. Pat. No. 4,813,391), since a fuel injection nozzle is mounted on a cylinder body and fuel is directly injected from the fuel injection nozzle into the combustion chamber, the discharge of HC in the exhaust gas due to the phenomenon of blow-by of unburnt air-fuel mixture from the combustion chamber, or the discharge of unburnt fuel due to the phenomenon of spitting of fuel toward the air-cleaner can be minimized. However, since the structure of the engine itself is formed as a two-cycle engine, the reduction in discharge of unburnt air-fuel mixture is not yet sufficient enough, thus leaving room for further improvement.
Furthermore, since the fuel is injected by the delivery pressure of a fuel pump from a fuel injection nozzle at the delivery motion thereof, the timing of fuel injection as well as the quantity of fuel to be fed are rather difficult to adjust, thus making this fuel injection mechanism unsuitable for controlling the air/fuel ratio, as in a lean-burn technique for instance.
Moreover, this prior technique is irrelevant to an improvement of certain phenomena peculiar to the two-cycle internal combustion engine, such as the rattling of the piston during idling or a loud combustion noise.